Heating, Ventilation, and Air Conditioning (HVAC) of building interiors may be accomplished through a variety of methods. One method is the use of a heat pump. In its most general form, a heat pump employs electrical energy to remove heat from a cool region and then delivers the heat to a warmer location. Generally, this is accomplished by conveying refrigerant at a low pressure and low temperature from an evaporator to a compressor. The compressor performs work on the refrigerant and transforms it into a high temperature vapor which is conveyed to a condenser. The liquid from the condenser is expanded into a mixture of cold liquid and vapor, and is then vaporized at the evaporator. The cycle is then repeated.
A heat pump may be used for either heating or cooling. This dual purpose thermal conditioning is generally accomplished by operating the heat exchanger as a condenser within the building for heating during the winter, and then operating it as an evaporator for cooling during the summer.
Generally, three types of heat pumps are commercially available. One type is a variable speed heat pump which, as its name implies, operates at continuously variable speeds, and thus enjoys a wide thermal output spectrum. Another commercially available device is a multiple speed heat pump. A multiple speed heat pump generally operates at a low speed or at a high speed. The third, and most common, type of heat pump is the single speed heat pump.
The desired thermal condition produced by the heat pump may be distributed throughout the building by the use of a thermal distribution system. An air distribution system is the most common. A hydronic system may also be used. In a hydronic system, circulated water distributes the thermal output of the heat pump. In such a system, a delivery conduit routes the liquid to a particular zone or region within the building. A heat exchanger is positioned at each zone. The ambient air within the zone is either heated or cooled by the heat exchanger to maintain the desired condition in the zone. After exiting the heat exchanger, the liquid is re-conveyed to the heat pump by a return conduit. The heat exchangers used in a building are typically fan coil units which include a finned heat transfer coil in close proximity with a fan which forces room air over the heat transfer coil. Other types of heat exchangers are also used in hydronic distribution systems.
It is generally desirable for a building to have as many independently controlled zones as possible. Multiple independently controlled zones permit a variety of thermal conditions. That is, an occupant may tailor the ambient condition in his or her own zone without impacting the conditions in another zone. In addition, multiple zone systems can maintain uniform conditions throughout a building, despite variations in the heating or cooling loads within zones. Such variations may be caused by differences in thermal insulation, the quantity or quality of heat generating appliances, or the number of occupants within a zone. Another advantage of a multiple zone system is that it can be energy efficient by isolating unused zones.
While multiple independent zones are highly desirable, they also introduce a number of thermal efficiency problems. In particular, the control unit must be responsive to widely varying demands of the independent zones. To successfully achieve this, the speed of the heat pump must be varied constantly. This can cause the temperature of the thermal transfer fluid to widely vary from its desired average or mean value. For the heat pump to operate most efficiently, or most usefully, its delivery temperatures in both heating and cooling must be closely controlled. Delivery temperatures too high in heating or too low in cooling cause inefficiency. Delivery temperatures too high in cooling and too low in heating cause discomfort.
In the heating mode, meeting the load may require supplemental heating. This heating is commonly delivered by electrical resistance heaters, but other sources, such as natural gas, may be used. For a heat pump to be efficient, the use of supplemental heating, especially if it is thermal resistance heating, must be minimized.